I recommend Gokhale method (https://www.amazon.com/Steps-Pain-Free-Back-Solutions-Shoulder/dp/0979303605 https://www.amazon.com/Steps-Pain-Free-Back-Solutions-S

I recommend Gokhale method (https://www.amazon.com/Steps-Pain-Free-Back-Solutions-Shoulder/dp/0979303605 https://www.amazon.com/Steps-Pain-Free-Back-Solutions-Should...) which helped me fix my back pain and allow to sit for long periods of time without any problem. This book will teach you how to sit correctly and few other things about your back.